Created a path finding algorithm

Created a path finding algorithm using DFS that generates a direction
map for enemies instead of having us write one manually. Also fixed an
error in Enemy class that wasn't allowing the enemies to move NW
added top menu (partially)
Merge branches 'master' and 'robert'
Merged branch "master" with branch "robert"
Changed enemy sprite

Derp
Merge remote-tracking branch 'origin/master' into Italo's
Towers now attack in an interval determined in milliseconds.
The right mouse button can now remove towers.
Fixed it so towers now properly attack after cooldown.
Changed target debug line to yellow from cyan.
Enemies now show their health, and transition from green to red as they get damaged.
Readded missing code to clear the screen.
Updated .gitignore to ignore gen/*/BuildConfig.java
Changed so Tower#inRange is declared once as opposed to being instantiated every Tower#findTarget call.
Changed TowerDefense#changeScreen to static, so that any class may modify the game screen.
Removed redundant code to initialize array.
Added limit to maximum number of towers that may be placed.
Merge remote-tracking branch 'origin/master' into Italo's
Implemented Spawn System.  Waves currently start as soon as the last one is killed.
Fixed Crashing after last wave